1. Technical Field
This disclosure relates to semiconductor fabrication tools and more particularly, to an extrusion enhanced mask for improving a process window for lithographic processes in semiconductor technology.
2. Description of the Related Art
Semiconductor fabrication processes typically include photolithographic processing to pattern areas of a surface of a semiconductor device. The semiconductor fabrication process typically includes applying a photoresist material to the surface of the semiconductor device. The photoresist is patterned by exposing the photoresist to light, typically ultraviolet light, to crosslink the resist material (negative resist). This cross linking prevents a reaction with a developer which develops away areas of the photoresist which were not crosslinked by the exposure to the UV light. Other types of photoresist have chains broken by exposure (positive resist) to ultraviolet light.
Photoresists are patterned using a photomask. The photomask functions as a shield to prevent light form passing through it in predetermined areas during photolithography. The photomask typically provides a black or highly absorbent layer of material, usually chromium or a chromium alloy, patterned in accordance with the patterning design to be projected onto the photoresist. The absorbent layer is formed on a substrate, which may include a glass or quartz material. Other techniques are used which may include electrons and electron beam masks, scattering masks and/or stencil masks, for example, scattering with angular limitation in projection electron beam lithography (SCALPEL).
With decreasing feature sizes of semiconductor components, masks are increasingly more difficult to fabricate and inspect. It is known that advanced semiconductor processing is very sensitive to image quality provided by masks. The defect fabrication capability for reticles is limited to a certain minimum feature size. This minimum feature size typically depends on the process and fabrication tools used to provide the pattern on the reticle.
Image quality is dependent on the resolution capability of the optical lithography process. When a photo mask is used, variations in a semiconductor wafer may affect the resolution. For example, wafer imperfections or size changes may vary from lot to lot. The resolution capability of an optical lithography system is generally defined as k.sub.1 .lambda./NA where k.sub.1 is a constant related to the process conditions, .lambda. is the illumination wavelength of the light shown through the mask, and NA is the numerical aperture of the optical lithography system. A depth of focus (DOF) is defined as k.sub.2 .lambda./NA.sup.2, where k.sub.2 is another process constant. From this relationship, it is clear that smaller feature sizes may be printed, but at the expense of the depth of focus. Depth of focus relates to the ability to focus the illuminating light on the semiconductor wafer which is to be process using photolithography. A larger depth of focus is desired since it provides easier manufacturing processes for lithography. The flexibility or tolerance afforded an optical system is described in terms of exposure latitude. The greater the exposure latitude the more robust the optical system. Smaller feature sizes, however, have smaller exposure latitudes. Due to a certain amount of randomness associated with manufacturing process, both a large depth of focus and exposure latitude are desired.
For manufacturing processes, the depth of focus and the exposure latitude are maintained within a predefined specifications. These specifications are achieved only for certain values of the depth of focus and the exposure latitude. The values of the exposure latitude and the depth of field should provide optical illumination within the manufacturing specifications. These quantities may be combined as a product which is referred to as the process window, i.e., exposure latitude times depth of focus. Alternately, an area below the curve of a plot of depth of focus versus exposure latitude may be used to determine the process window. This is known as a total process window, i.e., area under the curve=total process window. The process window may include bounds for providing optical illumination within the manufacturing specifications.
Therefore, a need exists for an apparatus for improving the process window such that optical illumination is provided within the manufacturing specifications over a greater range of values. A further need exists for increasing the depth of focus and the exposure latitude while maintaining or improving resolution for features to be imaged.